Deposit Formation in Muscle Fibers following Contraction in the Presence of Lead

The location of phosphatase sites within muscle cells is of interest since adenosinetriphosphate (ATP) hydrolysis is involved in (a) force generation by the myofilaments (Engelhardt and Ljubimova, 1939; Szent-Gy6rgyi 1953; Weber, 1958; Cain et al., 1962) and (b) the translocation of calcium from the myofilament space into the sarcoplasmic reticulum that controls relaxation (Ebashi and Lipmann, 1962; Hasselbach and Makinose, 1963; Weber et al., 1963; Podolsky and Costantin, 1964). The present report is based on experiments designed to localize such sites for microscopic study by applying the lead precipitation technique (Wachstein and Meisel, 1957) to a limited region of a "skinned" muscle fiber (Natori, 1954). One result is that these preparations develop electronopaque deposits associated with the sarcoplasmic reticulum, which appear to be unrelated to the transport of calcium. Another finding is a negative one which raises questions about the validity of ultrastructural localizations of phosphatase activity made with the lead precipitation technique; when the enzymatic reactions accompanying muscle contraction are activated by addition of calcium ions to the cell, the presence of lead in the contracting region does not necessarily produce myofilamentassociated precipitates.